This invention claims priority of a German patent application DE 100 21 379.0 which is incorporated by reference herein.
The invention concerns an optical measurement arrangement, in particular for layer thickness measurement and for ascertaining optical material properties of a specimen, comprising an illumination device for emitting a measurement light beam, a beam splitter for dividing the measurement light beam into a specimen light beam and a reference light beam, a measurement objective for directing the specimen light beam onto a measurement location on the surface of the specimen and for acquiring the light reflected from the measurement location, and an analysis device into which the reference light beam and the specimen light beam reflected from the specimen are coupled in order to obtain information about the specimen, in particular about layer thicknesses present thereon.
Optical measurement arrangements that operate on the principle of spectrophotometry, and their particular use for layer thickness measurement, are known in many varieties from the existing art. They have been utilized with particular success in the measurement of thin layers and optical parameters (such as refractive index n and extinction coefficient k) of single-layer and multilayer systems on patterned wafers.
Since increasingly fine patterns and thinner layers are desirable in particular in wafer manufacture, requirements are also increasing in terms of the accuracy of the optical measurement arrangements with which the dimensional consistency of the patterns and layers can be verified. In the context of this development, there is an interest in performing measurements both on single-layer systems and on multilayer systems. In addition, it should be possible to make exact measurements, with the same optical measurement arrangement, of thin layers having thicknesses of approximately 1 nm and above and of thicker layers up to approximately 50 xcexcm.
In addition to the demand for increased accuracy, the desire for greater production volumes must also be taken in account. In the continuous production of wafers, for example, it is necessary to measure them at shorter and shorter time intervals, and whenever possible in-line. On the other hand, space considerations must be also be borne in mind when integrating an optical measurement arrangement into a continuous production line. The arrangement should require little space, and should be flexible in terms of setup. Also desirable is suitability for rapid adaptation to changes in physical conditions. In this context, however, accuracy considerations must never be forgotten.
An optical measurement arrangement of the kind cited initially is known, for example, from U.S. Pat. No. 5,486,701. This is based on the principle of spectral analysis of the light reflected from a measurement location, each thickness having associated with it a characteristic interference-dependent reflection spectrum. The optical measurement arrangement disclosed in U.S. Pat. No. 5,486,701 is, however, specialized for the measurement of extremely thin layers. For that purpose, a separate analysis is made of the UV region and the visible region of the reflected light, so as to obtain therefrom the actual information concerning the measurement location on the specimen. Because of the large number of optical assemblies incorporated into the beam path, the assemblage is complex and sensitive to external influences. The optical deflection devices which transfer a measurement light beam emitted from the illumination device to the specimen and on to the analysis device further limit the design possibilities with regard to a compact assemblage.
The lateral resolution capability for patterns on the specimen depends on the wavelength of the measurement light used. Shorter-wavelength light allows finer patterns to be resolved. In connection with the measurement of very thin layers (in the range of less than one-tenth the wavelength of the measurement light), a further consideration is that the brightness differences that are to be analyzed, from which the information about layer thickness is obtained, become very small. With thin layers in particular, it is important to transfer the light reflected from the specimen to an analysis device with as little loss or interference as possible.
Against this background, it is the object of the invention to create an optical measurement arrangement that, with a simple and compact configuration, allows accurate information to be obtained concerning properties of a specimen being examined, in particular a patterned wafer.
This object is achieved with an optical measurement arrangement of the kind cited initially in which light-guiding devices having a plurality of light-guiding fibers are provided for coupling the specimen light beam and the reference light beam into the analysis device.
The arrangement at this concrete location of the light-guiding devices, known per se in the field of optical applications, makes possible a good compromise between minimally distorted transfer of the measurement lightxe2x80x94i.e. of a component uninfluenced by the specimen (namely a reference light beam) and a component influenced by the specimen (namely a specimen light beam)xe2x80x94and a particularly compact and at the same time flexible configuration of the optical measurement arrangement, which as a result is particularly suitable for being set up in continuous production lines.
Use of the light-guiding devices directly at the entrance of the analysis device allows the latter to be arranged flexibly with respect to the measurement objective. The arrangement is moreover insensitive to interfering influences from the production process, for example vibration.
Preferably a separate light-guiding device is used in each case for the specimen light beam and for the reference light beam, so that beam guidance is mutually independent and thus can be accomplished with as little restriction as possible in accordance with the particular physical conditions that are present.
In a further advantageous embodiment, the exit ends of the light-guiding fibers at the entrance of the analysis device are arranged in correspondence with a receiver provided on the analysis device. Optimum yield of the light reflected from the specimen is thereby obtained. In consideration of analysis devices that are common on the market, for example spectrographs with a downstream CCD detector, the exit ends of the light-guiding fibers are preferably arranged in linear fashion.
To achieve the clearest possible differentiation between maxima and minima in the spectrum of the specimen light beam reflected from the specimen, in a further advantageous embodiment of the invention an aperture stop is arranged in the measurement light beam in front of the beam splitter. The aperture stop preferably has a passthrough opening in the shape of a quarter-circle ring, hereinafter also called a xe2x80x9cquarter pupil.xe2x80x9d This allows the incident and return light to be guided separately to the measurement objective, and additionally permits the light of a focusing device to be coupled into the measurement objective in order to align the specimen relative to the measurement objective.
For optimum utilization of the light in the aperture, the entrance ends of the light-guiding fibers for the specimen light beam and the reference light beam can be distributed over a surface on which the shape of the opening of the aperture stop is reproduced. The enveloping curve of the light-guiding fibers corresponds substantially to the contour of the opening of the aperture stop. This advantageously also ensures that complete illumination of the entrance ends of the light-guiding devices remains independent of the size of a measurement window on the specimen, and of the settings of the field stop and the pinhole mirror.
It is preferable to use light-guiding devices in which the positional allocation between the entrance ends and exit ends of the individual light fibers is random. For particular measurement tasks, however, it may be advantageous if identical regions of the reference light beam and the specimen light beam can be compared to one another. For that purpose, for example, the positional relationship between the entrance ends and exit ends for each light-guiding device can be determined experimentally, and that positional relationship can be taken into account in the analysis of the signals received in the analysis device. It is also conceivable to arrange the entrance ends and exit ends of the light-guiding fibers for both light-guiding devices in the same positional relationship to one another.
It is at times of interest also to observe the wider surroundings of the measurement location, itself very small, on the specimen. In principle, this can be accomplished to a certain extent by exchanging the measurement objective, but this increases the outlay in terms of equipment for the actual measurement arrangement. The switchover devices which are then required can also degrade the quality of the specimen light beam. In an advantageous embodiment of the invention, therefore, a further objective is provided into which light of the illumination device can be switched for application onto an observation location on the specimen, the measurement light being coupled into the further objective by way of a transfer device separate from the measurement light beam. The almost complete separation, in terms of systems and equipment, between observation of the surroundings of the measurement location and observation of the actual measurement location makes possible a modular construction, so that the further objective with the associated transfer device can be configured as an optionally usable module. On the other hand, because the light used is that of the illumination device which is in any case always present on the optical measurement arrangement, the additional equipment outlay for this module is small. The further objective can of course also be used for additional examinations of the specimen.
The light captured by the further objective can preferably be coupled into a visual display device. With this, for example, the measurement operation can be observed online. It is also possible additionally to record the visually obtained information.
For the measurement of particularly thin layers, it is of interest to use a measurement light with the shortest possible wavelength, i.e. in the deep UV region. For spectrophotometry in this context, a perpendicular incidence of the illumination onto the measurement location of the specimen would be advantageous, since polarization effects could then be avoided. Such a situation, i.e. a very small numerical aperture, would yield a spectrum in which there would be no mixing of reflected light spectra of the light that had been applied at different angles of incidence onto the measurement location. A very small aperture means on the other hand, however, that only a very small quantity of light is available for measurement purposes.
Against this background, it has proven advantageous to use as the measurement objective a mirror objective having a numerical aperture of less than 0.3, and furthermore to use an illumination device that comprises a halogen lamp and a deuterium lamp. Under these conditions, interference spectra with sufficient intensity differences between the minima and maxima can be obtained even with very thin structures. Spectral imaging aberrations are greatly limited by the elimination of glass optical elements.